1. Field of the Invention
The present invention relates to an oscillator such as a voltage controlled oscillator for example, and particularly relates to an oscillator, a PLL oscillator, a radio apparatus suitable for a radio communication apparatus capable of using plural communication methods.
2. Description of the Related Art
For radio apparatuses, mobile radio apparatuses in particular, decreasing power consumption is one of major issues. For digital radio apparatuses, which are becoming common recently, decreasing power consumption of an analog part thereof contributes largely to decreasing power consumption of an entire radio apparatus.
Incidentally, as a digital radio apparatus, one adopting a method to switch not only transmission power but also a modulation method depending on a propagation environment is known. For example, IEEE 802.11a, which is one of wireless LAN standards, defines to use a modulation method which has high frequency utilization efficiency and is capable of high speed transmission, such as 16 QAM or 64 QAM, in an environment where few noises and interference signals exist and use a modulation method which has a slow transmission speed but has a wide signal point interval and is less affected by noises and interferences, such as QPSK, in an environment where many noises and interferences exist.
As a local oscillator of an orthogonal modulator-demodulator or a clock signal source for an ADC (A/D converter) in such a radio apparatus, normally a PLL (Phase Locked Loop) is used. In a radio apparatus with a variable modulation method in which a modulation method switches, a tolerable noise level varies according to a modulation method used, and therefore accuracy required for the PLL varies accordingly.
Here, the accuracy of the PLL means a phase noise level, and mainly, it is often expressed as a phase noise spectrum on a frequency axis in the local oscillator, and as a jitter on a time axis in the clock signal source for an ADC. For example, when a modulation method which has high frequency utilization efficiency, high accuracy is required also in the orthogonal modulator-demodulator and in the ADC, so that the phase noise level of the PLL needs to be lowered accordingly. On the other hand, when a modulation method which is strong to noise is used, the phase noise can be tolerated to a certain level, and thus power consumption of the PLL can be decreased.
PLLs capable of controlling the phase noise level are disclosed in, for example, JP-A 2004-364055 (KOKAI) (hereinafter referred to as “Document 1”) and in JP-A11-88164 (KOKAI) (hereinafter referred to as “Document 2”). Document 1 is a technique related to a PLL capable of using with plural communication methods, which is specifically for selecting from plural loop filters having different time constants an optimum one according to an oscillation frequency band of a VCO (voltage controlled oscillator). Thus, it is possible to optimize the phase noise level of the PLL for plural communication methods having different tolerable noise levels and frequency bands.
Further, in Document 2, the clock signal frequency of a decimal division control unit in a fractional synthesizer is changed according to a modulation method so as to optimize power consumption. This synthesizer performs decimal division control by means of a clock signal obtained by dividing a reference signal, and changes the frequency of a control clock by changing a dividing ratio for the reference signal. Accordingly, a loop gain of the PLL changes, so that the phase noise level can be controlled.
However, the method described in Document 1 is for optimizing the phase noise level for plural communication methods and is not capable of optimizing power consumption. Also, the loop filter is constituted of passive elements such as capacitors, and thus there arises a problem of increasing costs when plural filters are included.
Also, the method in Document 2 allows optimization of power consumption according to a modification method, but since most of power consumption in a PLL is consumed in an analog part such as a VCO, significant reduction effect in power consumption cannot be expected.
As described above, there is a problem in conventional oscillators, PLL oscillators and radio apparatuses such that the optimization of power consumption according to a modulation method or a propagation environment cannot be performed.